The International Liquid Mirror Telescope (ILMT) will be pointed at the stars every night for a period of five years. Its mission will be to study the photometric variability of all the objects present in a wide field of observation. It should make possible the detection of more than 9,000 quasars including gravitational lenses which are physical phenomena from which multiple images of a single source are visible to the observer due to the presence of a massive object near the line of sight of the source, deflecting the light from the source. What is the objective of the study? On one hand, to better understand the expansion of the universe as well as to constrain the fraction of the matter contained in it and, on the other hand, to study the mass distribution of the deflectors causing gravitational  lensing phenomena. An international collaboration to which a doctoral student from Liège contributed greatly, first by desigining an instrument to detect the wavelets that can possibly deform the surface of the mirror, and then by predicting the scientific input of the telescope.

The very least that can be said is that this is an innovative technology to which François Finet, an astrophysicist at the Institute of Astrophysics and Geophysics of the University of Liège has given his full attention by integrating the subject of his thesis with an international research project. He contributed to the development of the ILMT (International Liquid Mirror Telescope), a liquid mirror telescope with a diameter of 4 meters, and also made a prospective analysis on the scientific mission to be fulfilled by this instrument. The next step will be to assemble the instrument in India before calibrating it and beginning observations of the heavens. Even a brief glance at the ambitious program planned for this extraordinary instrument promises a bumper crop of stars. Before going into this project in detail, it would perhaps be useful to take a look at the characteristics of this technology which is not very well known to the public.

What is a liquid mirror?

It is difficult, when thinking about our bathroom or rearview mirror to imagine how a mirror could be liquid, yet the idea is almost distressingly simple. By rotating mercury, a reflecting liquid, on a circular dish with a perfectly vertical axis, we get a parabolic mirror. By inserting a sensor on the focal surface, the point where all the light rays captured by the mirror are reflected, a telescope is obtained.

This idea is based on the physical principles of a rotating liquid. A rotating liquid undergoes the action  of two forces. Gravity will exert a downward force, and the centrifugal force will exert a sideward force. At equilibrium, the surface of the liquid sets perpendicularly to the net force it undergoes. Because the centrifugal force is stronger the further we move away from the center, the surface of the liquid is therefore more tilted the further away you go from the center of the mirror. The resulting surface is that of a parabola.